Many pipes used for pipelines are manufactured by a welding process. The welds are usually girth welds or spiral welds. The pipes made this way have weld beads along the weld that protrude from the rest of the pipe surface. The internal surfaces of the pipes may also have corrosions. It is often necessary to prepare and clean the internal surfaces for subsequent coatings applications or to remove the corrosions.
Internal blasting is a method commonly used to prepare or clean the internal surfaces of the pipes. During internal blasting, abrasives are blasted toward the internal surface either by a mechanical turbine wheel or in a compressed air stream under high pressure. Commonly used abrasives include grit and shot blast media of various shapes and sizes, depending on the desired cleanliness and/or anchor profile necessary for satisfactory coating.
Conventional internal weld blasting systems normally blast at an impingement angle intended to facilitate blast efficiency and removal of residual abrasives from the target surface. Common conventional system uses a control cage with a single 70 deg opening, resulting in the abrasives spraying from the turbine wheel at a 70 deg angle which is directed so that the center of mass of the sprayed abrasives is slightly ahead of the blast wheel, wherein the center of mass impacts the weld, which provides adequate cleaning of the front side and top section of the weld but fails to clean the back side of the weld leaving an unblasted shadow. When a pipe contains a raised weld seam, a “shadow” is created on the leeward side of the weld that receives reduced blast exposure and as a result does not achieve the intended surface condition. This can result in failure to meet regulatory or client specification, and/or result in inadequate coating adhesion.